Abstract:

The linkage between relative sea-level change, shelf-edge architecture, and evolution of Maastrichtian basin-floor fans in the Washakie Basin, Wyoming, has been investigated at the scale of lobes, lobe complexes, and submarine fans using 630 wireline logs. The basin-floor fan deposits of two adjacent clinothems form lobate shapes on the toe of slope and basin floor. The earlier lobe complexes of the two clinothems are only weakly developed (from no deposition to up to 3.9 km3 respectively in Clinothems 9 and 10), indicating small volumes of sandy sediment delivered to deep water. The lobe complexes (up to 6.4 km3 of each lobe complex) of Clinothem 9 aggraded with fixed slope channels and without strong basinward or lateral migration (40–170 m aggradation, 4–8 km progradation with 4 km lateral shift) and did so in concert with a highly aggradational shelf edge (50 m/100 ky with 5.5 km progradation) during a period of interpreted relative sea-level rise. In contrast, the deep-water lobe complexes (up to 11.5 km3 of each lobe complex) of Clinothem 10 prograded continuously for 15–18 km on the basin floor (with 60–210 m aggradation) coeval with a flattish shelf-edge progradation (25 km/100 ky with 25 m aggradation) and an interpreted minimal sea-level rise or stillstand. The depocenters of lobe complexes in Clinothem 10 switched laterally (7–14 km) by compensational stacking and slope-channel avulsions. During the late development of both clinothems, the deep-water lobe complexes became smaller (up to 1.9 and 6.1 km3 respectively in Clinothems 9 and 10) or retreated concurrently with shelf flooding. Washakie Basin deep-water fans thus evolved through stages of initiation, aggradation or progradation, and retreat of lobe complexes. The submarine-fan growth stages of these deep-water depocenters were surprisingly well linked to coeval changes in shelf-edge trajectory between successive, ca. 100 ky maximum flooding events on the shelf. We suggest that the close linkage of lobe-complex stacking pattern with shelf-edge behavior was because the Washakie Basin formed under greenhouse conditions with a continuously high, Laramide sediment discharge to the deep-water fans while the feeder deltas were at the shelf edge, despite significant sediment reworking of shelf-edge deltas by waves and tides.